Semiconductor heterojunction lasers are usefully made by processes that include the epitaxial growth of successive layers of III-V materials of varying composition on a III-V substrate, typically, a gallium arsenide or indium phosphide substrate. Such layers will herein be understood to extend horizontally, and the axis perpendicular to the layers will be understood to extend vertically. Such lasers may generally be classified into two groups, namely edge-emitting lasers and surface-emitting lasers. Edge-emitting lasers emit laser radiation in a substantially horizontal direction, whereas surface-emitting lasers emit laser radiation in a substantially vertical direction. A surface-emitting laser is said to have a vertical cavity (and is thus referred to as a vertical-cavity surface-emitting laser, or VCSEL) if the optical cavity is bounded by a pair of horizontally-extending reflective layers.
Laser radiation can be coupled out of the tops or bottoms of VCSELs without the need for structures, such as 45.degree. mirrors, for turning optical beams. As a consequence, VCSELs are regarded as desirable for applications involving optically emissive arrays and for optical integration, where they are packed side-by-side with other components or devices and access to the side portions of the laser elements may be precluded.
As for most lasers, it is generally desirable for one of the reflectors bounding the laser cavity to have high reflectivity, approaching 100%, at the lasing frequency. The other reflector, which functions as the exit mirror, desirably has a reflectivity that is less than 100%, but still relatively high. For example, the reflectivity of the exit mirror typically lies in the range 95%-99.9%. One useful method for forming a reflective layer structure is to grow a sequence of thin layers of alternating composition such that constructive interference occurs between reflections from the various included interfaces. This will take place if, for example, the thickness of each thin layer is at least approximately equal to one-fourth the wavelength corresponding to electromagnetic radiation of the laser frequency propagating in that layer. The resulting strong reflection (at the laser frequency) is referred to as a Bragg reflection, and a structure comprising such alternating layers is referred to as a distributed Bragg reflector (DBR).
For a given choice of layer compositions and a given laser frequency, the reflectivity of a DBR can be adjusted by appropriate selection of the number of layers. Thus a DBR having a relatively large number of layers can be used for the mirror of greater reflectivity, and a DBR having a smaller number of layers can be used for the mirror of lesser reflectivity. Moreover, a DBR can be used to enhance the reflectivity of a metallized reflective surface. Thus, for example, the mirror of greater reflectivity may comprise a metal layer and a DBR contiguous to the metal layer. In such a reflective structure, referred to as a hybrid metal DBR reflector (or, simply, a hybrid DBR), a relatively small number of semiconductor layers may suffice to provide reflectivity near 100%.
Significantly, because the active layer of a VCSEL is short, the gain in the laser cavity is generally limited by the reflectivity of the reflective structures. (This point is discussed, for example, in J. Faist, et al., "Characterization of GaAs/(GaAs).sub.n (AlAs).sub.m surface-emitting laser structures through reflectivity and high-resolution electron microscopy measurements," J. Appl. Phys. 66(1989) pp. 1023-1032.) Thus, in order to assure desirable levels of laser performance (e.g., relatively low values of the threshold current), it is important to provide reflective structures of high reflectivity. In particular, it is generally desirable to provide layers in the Bragg structure that are of high quality, i.e., that are substantially planar and of substantially uniform thickness, and that form sharp interfaces with neighboring layers. Roughness of the layers, i.e., spatial variations in thickness or spatially varying deviations from planarity, is capable of scattering laser radiation and thus decreasing the reflectivity.
As is evident from the foregoing discussion, DBRs are desirable as reflective structures because the reflectivity of such structures is readily adjusted by appropriate design. DBRs are also desirable because they can be designed to offer relatively low series resistivity to the flow of electric current through the laser. A VCSEL incorporating a DBR structure and a hybrid metal DBR reflector is described, for example, in K. Tai, et al., "Room-temperature continuous-wave vertical cavity surface-emitting GaAs injection lasers," Appl. Phys. Lett. 55 (1989) pp. 2473-2475, which is hereby incorporated by reference.
Although relatively high reflectivity is readily achieved in a hybrid DBR using relatively few, e.g., less than ten, semiconductor layers, a greater number of layers is generally required to achieve similarly high reflectivity in a semiconductor DBR. Thus, for example, Tai, et al. report the use of 22 layer pairs in a DBR for a VCSEL. However, spatial instabilities have been observed in at least some cases when a thin layer of a III-V material has been grown on a substrate of a different III-V material. Such instabilities are capable of causing roughness in a DBR, and such roughness may even propagate and grow in severity in subsequent layers as they are added to the DBR. For example, FIG. 6 is a transmission electron micrograph of a DBR structure in which such roughness is clearly apparent after the fifth layer pair, corresponding, typically, to about 670 nm of total growth in the DBR. As is apparent from the figure, this instability has a wave-like, or zig-zag, appearance, when the DBR structure is viewed in cross section. Moreover, it is apparent that the "waves" grow in amplitude as succeeding layers are added. One undesirable result of this instability is to limit the reflectivity achievable in DBR structures. As a consequence of relatively low DBR reflectivity, the threshold current achievable in a VSCEL incorporating such a DBR may be limited to a relatively high, and therefore undesirable, value.
The zig-zag instability described above is not the only instability plaguing the growth of epitaxial III-V layers. Thus, for example, R. K. Tsui, et al., "Effects of substrate misorientation on the properties of (Al, Ga)As grown by molecular beam epitaxy," J. Appl. Phys. 58 (1985) pp. 2570-2572 (Tsui I), reports that a rough surface morphology can appear in (Al, Ga)As epitaxial layers grown by MBE, under certain conditions, on GaAs substrates, and that the roughness can degrade a subsequently grown GaAs layer. The authors observe that the roughness is substantially eliminated by intentionally misorienting the substrate by a small tilt angle. Later, it was reported in R. K. Tsui, et al., "Properties of Al.sub.x Ga.sub.1-x As(x.sub.Al approximately 0.3) grown by molecular-beam epitaxy on misoriented substrates," J. Appl. Phys. 59, (1986) pp. 1508-1512 (Tsui II), that for growth of Al.sub.x Ga.sub.1-x As (x approximately equal to 0.3) on a (near) (100) surface of the GaAs substrate at substrate temperatures of 620.degree. C. and 650.degree. C., the optimum tilt angle was 3.degree.-5.degree., directed toward a nearest (111)A (gallium) surface. The authors also reported that reducing the growth temperature tended to increase the tilt angle required to grow smooth layers. Similarly, H. Z. Chen, et al., "Effect of substrate tilting on molecular beam epitaxial grown AlGaAs/GaAs lasers having very low threshold current densities," Appl. Phys. Lett. 51 (1987) pp. 2094-2096, reported that the morphology of structures grown on GaAs substrates, and consisting of GaAs quantum wells sandwiched between AlGaAs layers, was improved when the substrates were tilted from (100) by 4.degree. toward a nearest (111)A surface.
Thus, various authors have demonstrated that by appropriately tilting the substrate, it is possible to reduce or eliminate one particular growth instability that causes roughness in GaAs-AlGaAs heteroepitaxial structures. Relatively high substrate temperatures have also been found to improve the layer morphology. In the case of layers grown by molecular beam epitaxy, tilting of the substrate was successful in smoothing the layers when the substrate temperature was greater than 600.degree. C.
However, it is significant that the particular instability involved is one that appears in the very first "inverted interface;" i.e., the very first interface between an underlying GaAs surface and an AlGaAs layer grown on the surface. As described, for example, in H. Z. Chen, et al., the tilting method has been successfully applied for mitigating that particular instability and, as a result, for growing improved laser structures of a type that involve no more than about four heteroepitaxial layers. This situation is quite different from that of DBRs intended for incorporation in VCSELs. As noted, relatively many layers, typically more than ten, are required to provide adequate reflectivity of a DBR for bounding the optical cavity of a VCSEL. Unlike the "inverted interface" instability described by Tsui I, Tsui II, and Chen, cited above, the zig-zag instability does not appear in the first layer, but rather in subsequent layers, and it may even increase in severity as layers are added. In fact, the zig-zag instability can appear even under growth conditions such that the first inverted interface is smooth, and it can appear under growth conditions such that the roughness of each relevant aluminum-rich layer is smoothed by an immediately succeeding gallium-rich layer. Thus, the zig-zag instability occurs under circumstances quite different from those described, e.g., by Tsui I, Tsui II, and Chen, and as a consequence, those publications offer no guidance as to how it might be eliminated.
Zig-zag roughness in many-layered DBR structures has, in fact, been reported by J. Faist, et al., "Characterization of GaAs/(GaAs).sub.n (AlAs).sub.m surface-emitting laser structures through reflectivity and high-resolution electron microscopy measurements," J. Appl. Phys. 66 (1989) pp. 1023-1032. In that work, growth, at a substrate temperature of 680.degree. C., of quarter-wavelength layers of AlAs was alternated with growth of quarter-wavelength layers of (GaAs).sub.9 (AlAs).sub.2 superlattice. The roughness was mitigated by inserting six GaAs wells in each AlAs layer, each well having a width of about 2.5 .ANG.. However, this technique may offer some disadvantages because, inter alia, a lower substrate temperature during DBR growth is desirable, as discussed below, and also because the Faist technique necessitates the use of relatively many shutters in the MBE reactor, complicating the equipment and the procedure.
As noted, certain problems are posed by the substrate temperature. That is, when gallium-containing layers are grown by epitaxial growth methods such as MBE and MOCVD, it has been observed that the gallium sticking coefficient decreases as the temperature is increased. As a consequence, the mole fraction of gallium in the resulting layers is difficult to control, particularly at temperatures substantially greater than about 630.degree. C. As a consequence, it is desirable to reduce the substrate temperature even further, for example to less than 600.degree. C. while preserving desirable, i.e., smooth, layer morphology.
Thus, practitioners in the field have long sought a satisfactory method for growing VCSELs that comprise DBR structures that not only incorporate ten or more layers of alternating composition, but are also substantially free of roughness. Practitioners have hitherto been unsuccessful in finding such a method that does not involve the use of extra shutters for, e.g., intercalating included GaAs monolayers within aluminum-rich layers, and they have hitherto been unsuccessful in finding such a method that can be practiced at substrate temperatures, during DBR growth, of less than about 600.degree. C.